In this article we will discuss about Golgi Complex:- 1. Discovery of Golgi Complex 2. Function of Golgi Complex Including Processing and Transport of Secretory Product 3. Mechanism of Transport of Proteins From the ER to the Golgi Complex 4. Processing of Proteins in the Cisternal Space.
Discovery of Golgi Complex:
Camillo Golgi (1843-1926), an Italian cytologist, discovered some stained regions in the cytoplasm surrounding the nucleus after staining the cells with Osmium Tetroxide. These stained regions were termed Golgi body. But before the advent of electron microscope, these staining regions were thought to be an artifact.
Under electron microscope it has been observed that these regions under light microscope shows flattened membranous vesicles. These are not known as Golgi complex (dictyosome in some plant cells). The number, shape and location of the Golgi complex vary in different cell types. But usually they are present near the nucleus.
Thus the Golgi complex consists of a stack of flattened sacs bounded by smooth membranes. These sacs are surrounded by a number of spherical vesicles which help to transport proteins to and from the Golgi complex.
The stack of the Golgi complex has two definite regions cis and trans, the end of the stack receiving the vesicles from ER is called the cis region, whereas the opposite side of the stack facing the plasma membrane is the trans region (Fig. 3.4). So, the cis region is called the outer surface or convex face and the trans region is called the inner side or concave side.
Golgi complex or body is often called the traffic police of the cell controlling the proteins and vesicles to proper destinations. The function of this ‘Golgi complex is done by a number of enzymes present within it. The most important and unique enzyme found in the Golgi complex is Thiamine pyrophosphatase which can be used as an identifying character of the Golgi complex.
There is another unique enzyme, Nucleoside di-phosphatase, in the Golgi complex. This enzyme may be present in some cases in the endoplasmic reticulum. Again, the enzymes like Glucose-6-Phosphatase and NADPH cytochrome P450 reductase are absent in the Golgi vesicles while these are present in the endoplasmic reticulum.
Thus these membrane elements can be cytochemically and chromato- graphically distinguished. The distribution of enzymes in Golgi vesicles is heterogeneous—depending on their function.
Some enzymes are found in higher concentration at the two ends than at the central part of the sacs. Biochemical studies have been done on Golgi components by isolating and purifying Golgi membranes through zonal and iso-density centrifugation.
One important function of Golgi enzyme is to add a ‘group’ such as carbohydrate or phosphate residue to certain proteins to their specific destination in the cell. For example, the enzyme Glycosyltransferases help in attaching sugars to protein. Another enzyme, Glucan synthetizes, helps in carbohydrate metabolism.
Function of Golgi Complex Including Processing and Transport of Secretory Product:
The Golgi complex is mainly connected with the process of cell secretion. Using radioactive amino acid (H3) leucine the secretion of digestive enzymes by exocrine cells has been investigated. High resolution autoradiography through electron microscope showed, after a few minutes of exposure to H3 leucine, radioactivity in the rough endoplasmic reticulum.
Within 10 minutes the radioactivity was noted in the outer layer of the Golgi complex, at 30 minutes it was found within the Golgi vesicles.
After one hour, the radioactivity was mostly noted in the large vesicles on the trans face of the Golgi complex which were ready for secretion. This result shows that proteins are produced on the rough endoplasmic reticulum, then translocate to the Golgi complex, then to the secretory granules.
These granules finally secrete the contents out of the cell. In another experiment, cells are exposed to radioactive amino acids for a brief period and these cells are fractionated through centrifugation at different periods of time.
Analysis of radioactivity was done through Liquid scintillation counter which shows the presence of radioactivity first in the endoplasmic reticulum (Rough), followed by the membrane of Golgi complex and, finally, to the secretory granules or zymogen granules.
Mechanism of Transport of Proteins From the ER to the Golgi Complex:
It has already been mentioned that secretory proteins or polypeptides are synthesised on the ribosomes attached to the endoplasmic reticulum. This protein is then budded off from the endoplasmic reticulum in the form of transitional vesicles.
These vesicles then move towards the Golgi complex and fuse with cis region of the Golgi complex. Thus the product of ER is transferred to the cisternal space of the Golgi complex. This translocation of protein occur independent of accumulation of proteins in the cisternal space developing a concentration gradient.
This is because the process of transport continues even after the use of inhibitors of protein synthesis. But when synthesis of ATP is inhibited, the translocation of proteins from the ER does not take place showing the presence of energy mediated process.
Processing of Proteins in the Cisternal Space of the Golgi Complex:
Although most of the proteins transporting into the cisternal space of the Golgi complex are to be secreted out of the complex from the trans face, many ER membrane proteins also enter the Golgi sacs that are not destined for secretion.
These membrane proteins are then returned back to ER from the cis region of the Golgi complex which has been confirmed by noting the presence of these proteins only in cis region and not in trans region of the Golgi complex. The stacking arrangement of the Golgi bodies actually facilitates this process of screening or sorting.
Proteins passing out through the trans region for secretion become concentrated into granules generally at the end of the Golgi sacs or near the margin. These concentrated proteins then form a membrane-enclosed vesicle budding off from the Golgi sacs.
These small vesicles may fuse to form a large vesicle sometimes termed as Zymogen granules. The mechanism for concentrating the proteins in the Golgi complex is through the binding of Peptidoglycans with the protein forming an insoluble aggregate.
The formation of such insoluble aggregates lead to lower the osmotic concentration of protein molecules resulting in the exit of water from the vesicle to the cell sap. In this way protein is being concentrated within the vesicle or at the margin of cisternal space of the Golgi complex.
In addition to concentrating the proteins, the Golgi complex has another important role of modifying the proteins by adding either fatty acid residues or carbohydrate groups to form lipoproteins or Glycoproteins. The protein glycosylation reactions are carried out through the enzymes—Glycosyltransferases—present in the Golgi bodies.
Sometimes the sugar residues are attached to protein chains in the ER, while more sugars are terminally added to the protein chain after transporting them to the Golgi complex. These proteins bounded by membrane come out of the Golgi complex and are either stored as zymogen granules till a suitable signal (hormone etc.) helps to remove them from the cell.
Sometimes these vesicles are discharged immediately from the cell. Some authors think that microtubules may indirectly control the discharge of vesicles or secretory product from the cell.
When the vesicle or secretory granule originating from the Golgi complex reaches the cell surface, the fusion of the membrane of the vesicle and plasma membrane takes place. The membranes break open at the point of fusion releasing the contents of the vesicle into the external medium. This process is known as exocytosis.
The membrane-barrier of the cell remains intact during this process. Sometimes the membrane of the vesicle, after exocytosis, migrates back to the Golgi complex. Thus all plant cells may be called secretory cells because they release polysaccharides, proteins etc. outside the cell membrane for the formation of cell wall.
In case of animals, most of the cells release hormones, enzymes, neuro-transmitting substances, plasma proteins, antibodies etc. and some fibrous materials like collagen and elastin. The Golgi complex in those cells helps in the formation of lysosomes, vesicles or certain specialised granules within the cell which do not perform any secretory role.
Membrane Recycling of Secretory Cells:
As the secretion of proteins etc. from the vesicle in the cell takes place through exocytosis, it causes considerable expansion of the plasma membrane. So, the cell must either degrade the extra membrane components or recycle them by endocytosis to maintain a constant membrane area.
The bulk of these membrane components is recycled to the region of the Golgi complex where they fuse with the associated membranes.
Evidences for Membrane Recycling:
Recycling of membrane was confirmed through an analysis of the binding of Immunoglobins (IgG) to the cell membrane in rabbit using a radioactive or fluorescent label. This membrane complex (IgG) was first noted in the vesicular membrane and then into the surface of the plasma membrane.
The pathway of this translocation or recycling of membrane was noted through immuno-fluorescent technique. Willingham et al (1984) studied the recycling process of the membrane by comparing the internalisation of epidermal growth factor (EGF) and transferrin.
Both EGF and Transferrin are inserted into the cytoplasm through endocytosis forming vesicle. EGF is transferred to the lysosome through some of its degradations.
Transferrin is transferred to Golgi system and from the latter both transferrin and transferrin receptors are transferred to the plasma membrane through dumb-bell shaped extensions of the Golgi complex. This shows that Golgi complex has an important role in targeting any molecule.
This process of recycling is helpful to deliver materials quickly inside the cell. It is also helpful in ingesting food materials by Amoeba. It can also help in trans-locating materials from one compartment to another. It also trans-locates receptor from one extra-cellular compartment to a cell surface or another cellular compartment.
This clearly shows the dynamicity of the membrane. However, several types of cytoplasmic transport are also associated with microtubules. There are microtubule- membrane interactions in the translocation of membranous organelles or some other cell particles.
This has been confirmed by observing some binding of microtubules with mitochondria, presence of microtubule organisation for continued secretion of insulin by pancreas cells.
The relationship between microtubule and mitochondria has been established in noting the presence of Golgi apparatus near the vicinity of perinuclear microtubule organising region in a human cell strain (A549). The interaction of these two systems have been confirmed by severed methods of dis-organisation and re-organisation of microtubules and Golgi apparatus.
Using single microtubules under in vitro systems, it has been found that vesicles can be transported along the microtubules in both directions. This bidirectional movement must depend on some factors.
One factor has been established as dynein, a protein that converts energy derived from hydrolysis of ATP into mechanical work, as in flagella and cilia. Other factors have been identified as proteins of tubulin group, Kinesin etc.